This invention relates to mechanical rotary vacuum pumps and, more particularly, to improvements in the general operating parameters for such pumps.
Rotary vacuum pumps of the type having a rotor mounted for rotation in a stator body and being offset in relation to the stator bore such that two blades contained in, and slidable within, diametrically opposed slots in the rotor can cause gas entering the space between the stator and the rotor to be compressed and expelled into a pump outlet.
Such pumps are typically provided as either a single stage version in which a single rotor is employed or a two-stage version in which a first (high vacuum) stage is backed by a second (low vacuum stage) with the exhaust gases from the first stage being directed, normally via a one-way valve, to the second stage. Generally, the capacity of the high vacuum stage is substantially higher than that of the low vacuum stage, for example by a factor of 4:1.
One stage and two stage pumps are known to possess different operating characteristics. For example, currently available one stage pumps are generally known to possess the ability to handle exhaust stream having a higher vapor content and to have a high throughput. Currently available two-stage pumps, although being able to achieve lower pressures, generally possess inferior vapor handling characteristics and have a lower throughput overall.
Ballasting of the pumps in question is a well-documented technique which enables the pump to cope with a certain amount of vapor without contamination of the pump oil. A gas ballast facility allows atmospheric air (a dry or inert gas can alternatively be used) to be admired to the pump chamber during the compression stage of the rotating blades. This increases the proportion of non-condensable gas in the pump and consequently the partial pressure of the vapor being pumped at the exhaust stage of the pump does not exceed its saturated vapor pressure; the vapor can therefore be discharged without condensing.
Known two-stage pumps tend to have the facility to be ballasted only in the low-vacuum stage as ballasting of the high-vacuum stage can jeopardize the vacuum in the system being evacuated. Equally, the injection of new lubricating oil is also supplied only to the low-vacuum stage. Such ballasting and lubricating features lead, however, to the performance characteristics described above.
In certain circumstances, it would be desirable to provide a two stage pump having as many of the beneficial characteristics as possible of both one and two-stage pumps.